In high-frequency/large-capacity optical fiber communication systems, optical modulators incorporating waveguide type optical modulation elements are widely used. Among them, an optical modulation element using LiNbO3 (hereinafter, also referred to as an LN) having an electro-optic effect on a substrate has been widely used for high-frequency/large-capacity optical fiber communication systems because optical modulation characteristics with a small light loss and a wideband can be realized.
In the optical modulation element using the LN, a Mach-Zehnder type optical waveguide, an RF electrode for applying a high-frequency signal as a modulation signal to the waveguide, and a bias electrode for performing various adjustments in order to maintain satisfactory modulation characteristics in the waveguide are formed. These electrodes formed in the optical modulation element are connected to an external electronic circuit via lead pins or connectors that are provided in a housing of the optical modulator including the optical modulation element.
On the other hand, modulation forms in optical fiber communication systems have influenced by a trend to increase transmission capacity in recent years, and transmission formats obtained by applying multi-level modulation or polarization multiplexing incorporated into the multi-level modulation such as a Quadrature Phase Shift Keying (QPSK) and a Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK), and the like have been mainstreamed.
Since an optical modulator (QPSK modulator) that performs QPSK modulation or an optical modulator (DP-QPSK modulator) that performs DP-QPSK modulation includes a nested Mach-Zehnder type optical waveguide, and has a plurality of high-frequency signal electrodes and a plurality of bias electrodes (see, for example, Patent Literature No. 1), the size of device tends to increase, and there is a strong demand for miniaturization in particular.
As a countermeasure against this demand for miniaturization, in the related art, push-on type coaxial connectors that are provided in the housing of the optical modulator as an interface for connecting an RF electrode and an external electronic circuit, have been replaced with the same lead pins as those of an interface of a bias electrode, and the optical modulator with a flexible printed circuits (FPC) added for connecting these lead pins to an external circuit board has been realized.
For example, in a DP-QPSK modulator, there is used an optical modulation element including four Mach-Zehnder type optical waveguides each having an RF electrode. In this case, since four push-on type coaxial connectors are provided in the housing of the optical modulator, it is inevitable to increase the size of the housing. However, it is possible to realize miniaturization by using lead pins and an FPC instead of coaxial connectors.
Since the lead pins in the housing of the optical modulator and the circuit board on which an electronic circuit for causing the optical modulator to perform a modulation operation is mounted are connected via the FPC, there is no need to use a coaxial cable used in the related art and a space required for the surplus length processing of the coaxial cable and it is possible to reduce a mounting space of the optical modulator in the optical transmission apparatus.
The FPC used for the optical modulator is manufactured by using, for example, a flexible polyimide-based material for a substrate (hereinafter, referred to as an FPC substrate), and a plurality of through-holes formed near one end portion are electrically connected to the same number of pads formed on an other end portion. A plurality of lead pins, which protrude from a bottom surface or side surface of the housing of the optical modulator, are inserted through the plurality of through-holes, respectively, and the lead pins and through-holes are fixed with solders. The plurality of pads are fixed to the circuit board with solders, respectively. As a result, each of high-frequency signals given from pads on the circuit board is, via the corresponding via through-holes and lead pins, given to the corresponding RF electrode of the optical modulation element, and high-frequency optical modulation is performed.
In the optical modulator using the FPC, as described above, the housing may be miniaturized and the mounting space of the optical modulator on the circuit board may be reduced, so that it contributes greatly to miniaturization of the optical transmission apparatus.
FIGS. 16A, 16B, and 16C are views showing a configuration of an optical modulator in the related art, including such an FPC; FIG. 16A is a top view of the optical modulator; FIG. 16B is a front view of the optical modulator; and FIG. 160 is a bottom view of the optical modulator. This optical modulator 1600 includes an optical modulation element 1602, a housing 1604 that houses the optical modulation element 1602, a flexible wiring board (FPC) 1606, an optical fiber 1608 for inputting light to the optical modulation element 1602, and an optical fiber 1610 that guides the light output from the optical modulation element 1602 to the outside of the housing 1604.
Four lead pins 1620, 1622, 1624, and 1626 connected to the four RF electrodes (not shown) of the optical modulation element 1602, respectively, are provided in the housing 1604, the lead pins 1620, 1622, 1624, and 1626 are inserted through through-holes 1720, 1722, 1724, and 1726, as described later, provided in the FPC 1606, and the lead pins and through-holes are fixed with solders.
FIG. 17 is a diagram showing a configuration of the FPC 1606. In the FPC 1606, four pads 1710, 1712, 1714, and 1716 are formed side by side in the vicinity of one side 1700 on a lower side in the figure along a direction of the one side 1700. On the side of an other side 1702 opposite to the side 1700, for example, four through-holes 1720, 1722, 1724, and 1726 are formed side by side along a direction of the side 1702. Further, the four pads 1710, 1712, 1714, and 1716 are electrically connected to the through-holes 1720, 1722, 1724, and 1726 by wiring patterns 1730, 1732, 1734, and 1736, respectively.
The four pads 1710, 1712, 1714, and 1716, respectively, are soldered to the pads of the external circuit board, thereby electrically connecting the RF electrodes of the optical modulation element 1602 included in the optical modulator 1600 to an electronic circuit configured on the circuit board, and mounting the optical modulator. Generally, a shape of the FPC 1606 is a horizontally elongated rectangle having a short side in a signal transmission direction as shown in the figure so as to shorten the wiring pattern as short as possible to suppress microwave loss, and in the case where the FPC 1606 has four pads 1710, 1712, 1714, and 1716 as in the shown example, the shape of the FPC 1606 is a rectangle having a length of about 20 mm or less in a long side direction and a length of about 10 mm or less in a short side direction.
FIGS. 18A and 18B are diagrams showing an example of a state in which an optical modulator 1600 is connected to a circuit board on which an electronic circuit is formed; FIG. 18A is a top view of the optical modulator 1600; and FIG. 18B is a sectional view taken along an arrow B-B in FIG. 18A. In FIG. 18B, the description of the internal configuration of the optical modulator 1600 will be not repeated.
The optical modulator 1600 and the circuit board 1800 are fixed to, for example, a base 1802 in the housing of the optical transmission apparatus. The FPC 1606 of the optical modulator 1600 extends leftward in the figure from a connection portion with the lead pins 1620, 1622, 1624, and 1626 and is directed obliquely and bent downward to the left in the figure of FIG. 18B, and the pads 1710, 1712, 1714, and 1716 in the FPC 1606 are soldered to the pads 1810, 1812, 1814, and 1816 on the circuit board 1800.
However, when the optical transmission apparatus is configured such that the optical modulator with the FPC as described above is connected to the circuit board, slight variation (variation in the thickness of intervening solders, variation in uniformity of the thickness, and variation in positional deviations between the FPC pads and the circuit board pads) may be generated in a connection state between the pads (FPC pads) 1710, 1712, 1714, and 1716 on the FPC 1606 and the pads (circuit board pads) 1810, 1812, 1814, and 1816 on the circuit board 1800, due to a deformation of the FPC 1606 generated in manufacturing of the FPC 1606, and various deformations such as warping and elongation of the FPC 1606, and the like generated when the FPC 1606 is soldered to the lead pins 1620, 1622, 1624, and 1626 in the housing 1604 of the optical modulator.
FIG. 19 is a diagram showing an example of a deformed FPC. In the shown example, the FPC 1900 is deformed upward in the figure at the corner portions of the substrate. Such deformation is commonly seen as a deformation caused by a manufacturing process of the FPC. For example, the FPC includes a wiring pattern formed of copper (Cu), gold (Au), or the like on a polyimide substrate, and further includes a protective film for protecting these wiring patterns and the like. Individual FPCs are manufactured by, for example, repeating and forming a wiring pattern configuring one FPC on a sheet-like base material of an FPC board in a matrix form at a plurality of times, and punching a portion configuring one FPC from the sheet-like base material. It is considered that the deformation at the corner portions as shown in the figure is derived from mass-production of individual FPCs due to punching as described above.
However, the deformation of the FPC generated at the time of manufacturing is generated due to various factors in the process of manufacturing the FPC, and the degree of the deformation may be changed due to factors such as material lot and production lot, and the like the deformation also has various shapes such as warping, undulation, elongation, and the like, and it is difficult to control and suppress them.
Further, such deformation of the FPC is generated not only by a manufacturing process, but also by heat generated when the FPC is soldered to the lead pin provided in the housing of the optical modulator, stress caused by stress applied to the FPC when the optical modulator is incorporated in the optical transmission apparatus, or stress applied to the FPC when the optical modulator is mounted on the circuit board.
Particularly, in the optical modulator, since the high-frequency signal propagated to the FPC reaches the microwave region of several tens of GHz, due to slight variation in the connection state as described above, large variation in reflection characteristics and transmission characteristics of a high-frequency signal path from the circuit board pad to the lead pin may be generated. As a result, it may be difficult to secure desirable optical transmission quality while maintaining satisfactory optical modulation characteristics in the optical modulator.
This problem may occur with comparative ease, caused by followings. Since the FPC pads have as small a size as possible due to the demand for miniaturization of the optical modulator, deformation of the FPC is likely to be generated by stress and the like applied to the FPC board during a manufacturing process of the FPC (for example, a punching process of the board), and since the FPC pads are very small, the FPC pads are likely to have positional deviations when they are soldered to the circuit board, and the like.
As a technique for solving such a problem, in the related art, particularly, in order to solve the above problem caused by that the FPC board is deformed into a U shape by its own weight, there is known an optical module in which, in order to deform the FPC into an inverse-U shape, a raised portion is provided on a portion of the housing on which the FPC abuts (Patent Literature No. 1).
However, even when the FPC is deformed into an inverse-U shape, it is difficult to manage various deformations such as warping and elongation, and the like of the FPC, which may be generated in an assembly process of soldering between the lead pins in the housing of the optical modulator and the FPC, or manage the deformation in manufacturing of the FPC itself in a constant state. Due to variation in processing of the raised portion provided on a portion of the housing, variation in deformation of the FPC may be also generated when the FPC is caused to abut on the raised portion. Therefore, the technique of the related art is limited to maintaining satisfactory optical modulation characteristics of the optical modulator by reducing various variation in manufacturing as described above. Further, in the technique of the related art, since a processing step of providing a raised portion on a portion of the housing is required, a manufacturing cost also increases.